Had the same sorts of problems with an f4 scope and made up a small additional tool to see what was going on. This made it easy to see mis-alignments that were not picked up in collimation. Most users don't seem to need this sort of approach, but it can help if you're unlucky enough to get a scope with a fundamental offset in something.
http://www.iceinspace.com.au/forum/s...ad.php?t=82230
I found that a 12inch GSO Newt needed some mods to work as a planetary scope.
http://www.iceinspace.com.au/forum/s...ad.php?t=85990 Agree with Zane re mirror cooling - even a degree difference between mirror and air can mess up the image at high mag. What you describe re star shape sounds like astigmatism - that will probably be due to the main mirror edge support.
Good luck, Ray